A power generator typically includes a stator, a rotor that turns within the stator to generate electrical power, and a mechanically driven shaft to turn the rotor within the stator. The rotor includes a field winding, to which an exciter current is supplied in generating electrical power. The exciter current typically is supplied with an exciter.
The exciter itself can be an alternating current generator driven by the same source (e.g., steam-powered turbine) as the rotor and feeding the direct current to the field winding of the rotor via brushes and slip-rings. Increasingly, however, “brushless” exciters are being used in electrical generators. With a brushless exciter, the DC current is derived from an alternating current. The alternating current is generated in the winding or armature placed on a shaft connected to the rotor, and a rectifier circuit converts the resulting alternating current into the direct current that is supplied to the field winding of the rotor.
An exciter includes a shaft, an armature connected to the shaft, a field surrounding the armature to generate an alternating current, and a rectifying wheel connected to the shaft. The rectifying wheel, moreover, typically comprises a plurality of semiconductor switching devices arranged so as to rectify the alternating current generated by the armature rotating within the field. The semiconductor switching devices can be, for example, a diode that conducts current when forward-biased.
A common problem associated with conventional rectifying circuits is commutation voltage spikes. These voltage spikes stem from turn-off transients associated with the use of semiconductor switching devices, such as diodes. As a rectifying diode transitions from a conducting state to a non-conducting state, a reverse recovery current is produced. The reverse recovery current can damage the semiconductor switching device. Moreover, were such damage to cause a diode failure, a phase-to-phase fault could result in destructively high currents within the armature windings. This danger is particularly acute in large-scale electrical generators in which high power density brushless exciters are typically used.
One conventional approach to reducing the level of voltage spikes during the transition of the rectifying diodes is to couple a snubber circuit to the rectifying diodes. Such a circuit typically includes a capacitor, resistor and fuse, with the capacitor being used to absorb stored charge during reverse recovery. Such circuits can be costly to produce and take up space within an electrical generator while adding weight to the exciter.
Another approach is found in U.S. Pat. No. 5,093,597 to Hughes, which discloses a saturable reactor diode snubber assembly that includes a thin film membrane composed of a saturable magnetic material. When a rectifying diode is conducting current, the saturable reactor diode snubber is in saturation, thereby exhibiting low impedance to the current. In transitioning to a non-conducting condition, however, the magnetic film of the saturable reactor loses its saturation and exhibits linear properties as the current approaches zero. Thus, when the diode transitions to a non-conducting state and the reverse current recovery condition arises, the saturable reactor diode snubber provides a high impedance to thereby reduce reverse recovery peak current and diode-stored energy.
One problem with such a device is that the saturable reactor will become extremely hot unless cooled, as, for example, by forced air. When natural convection cooling is applied, the temperature rise associated with the saturable reactor diode snubber is too high for operation (e.g., up to 120° C.). U.S. Pat. No. 5,731,966 to Liu attempts to deal with the problem by adding a resetting circuit that provides an additional conductive path coupled to a saturable reactor. The additional circuit, however, increases the complexity of the snubber circuit and thus increases the risk of a potential failure of the system.
Still another approach is that of U.S. Pat. No. 5,532,574 to Wolfe et al., which relies on a protection circuit that monitors the output voltage of a diode bridge rectifier and interrupt its operation if the voltage is not within an acceptable range. Such additional circuitry, however, suffers from many of the same problems associated with other conventional devices. For example, the additional circuitry adds weight to the exciter and uses additional space within the limited confines of the electrical generator, all while increasing the number of costly components of the exciter, increasing its complexity and, accordingly, its potential for malfunction.